Picking Optimal Altitudes

"O. Sami Saydjari" wrote in
:

When planning a flight against the wind, how to you pick the best
altitude when trying to minimize flight time?

If I read my performance charts correctly, my aircraft (Piper Turbo
Arrow III -- service ceiling 20,000 ft) seems to gain about 2 nts of
*true* airspeed for every 1000 feet you go up.

That's right... 2 knots increase in TAS per 1000 feet MSL is a rough
guideline, but good enough for calculations. Note that this only holds
true, however, for altitudes where you can hold the same power output.
This means, all other things being equal, that the most efficient
altitude is the maximum one at which you can still output your desired
power. For a normally aspirated aircraft flying at 75% or 65% power, we
are looking at between 5500 and maybe 8500 feet MSL.

For your turbo Arrow it depends on the condition of your turbocharger
and whether you have a Merlyn wastegate or not. With a Merlyn you can
easily hold 75% well up into the flight levels.

So, unless there is some other factor, I think this means that if the
winds increase at anything higher than 2 nts per 1000 feet, I am best
off staying at the Minimum Enroute Altitude. Is that right?

Right, but only a small part of the story. How bad is the thermals and
other turbulence at the altitude you want to fly. How hot is it? [In
Texas we often see 80+ degrees at 10,000 or higher.] Are you out
sightseeing, or trying to get somewhere. Icing? Thunderstorms around
(and do you have 'sferics equipment on board). How far are you going
(is it worth the time/fuel to climb)? Other reason to want to climb
high? [Like bad terrain at night.] Do your passengers like sucking on
an O2 hose?

And of course, which way is the wind blowing!!!

Lots of things to consider, and often not just one "right" answer.

Purely my own habit (also in a turbo Arrow III) I will fly anything over
an hours trip at least around 10,000 unless there is a big reason no to
(high winds). Gives good speed and ride (usually) without too much
time-to-climb. Over that, especially if by myself, I will climb up to
the O2 levels (again, depending upon wind). Long trips - ALWAYS go high
unless you are really fighting a massive headwind.

This time of the year, flying from Austin to El Paso (450 nm west of
here) in the winter (winds almost always out of the west up high) - I
will fly out there (west) at around 10K. That gives me acceptable
headwinds, while still keeping me high enough over the Davis mountain
range to avoid most turbulence. Coming back I will climb into the
flightlevels, pull back the power, and look for a 75+ knot tailwind.
[One trip I had a nice 250 - 275 knot groundspeed, on 55% power!!!]

BTW, some years ago I put all my flight characteristics into a small
computer program and just sat there playing with the results. For a
flight of X miles was it faster to climb to Y altitude, etc. Taking
into account the slower climb, the faster cruise, and the glide -
figured time and fuel. [Remember, this is always a no-wind simulation,
since it was a hypothetical trip.]

The results was not what I expected. Turned out that for anything much
over about 50 miles higher was pretty much always better. Time was
shorter the higher you went (descent doesn't buy back all the time you
lose on the climb, but it gets back a LOT of it). Fuel burn was
minimized at the highest altitude where you had anything over 1/3rd of
your time in cruise.


--
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James M. Knox
TriSoft ph 512-385-0316
1109-A Shady Lane fax 512-366-4331
Austin, Tx 78721
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